Ensign was the lowest rating of officer within most space navies, being bestowed upon a being immediately after graduating from a service academy. Ensigns typically were given supervisory positions over enlisted crew, however they were too junior to actually have a command billet. Ensigns also could not take command of any situation, unless in the absence of a lieutenant or other superior officer. Persons of this rank were typically assigned as watch officers, military aides, envoys or even bodyguards for various higher-ranking officers such as admirals. Ensigns also sometimes led ground missions, leading a vessel's complement of troopers on whatever task needed doing.
During off-duty hours, or when in hyperspace, an ensign could be given command of a warship, primarily to test their abilities, but also to give the regular officers some time off. When the captain or another higher-ranking officer judged them ready, the ensign would be promoted the next appropriate grade, and continue on their career path.
- Ensign on Wikipedia